Method of making thin-walled sintered metal articles



Jan. 30, 1945. w ENGLE 2,368,458

METHOD OF MAKING THIN WALLED SINTERED METAL ARTICLES Filed June 19, 1945PA PER Inventor:

Edgar Engle,

by jw w H is Attorney.

Patented Jan. 30, 1945 METHOD OF MAKING THIN-WALLED SINTERED METALARTICLES Edgar W. Engle, Huntington Woods, Mich., as-

signor to Carboloy Company, Inc., Detroit. Mich., a corporation of NewYork Application June 19, 1943, Serial No. 491,473

6 Claims.

The present invention is a method for manuacturing thin walled sinteredarticles and more particularly thin walled cemented carbide articles.Cemented carbides are sintered hard metal compositions containing one ormore hard metal carbides and a cementing metal therefor, such as cobalt.Such compositions generally are pressed into a desired shape andthereafter sintered into a hard dense mass. However. when employingconventional methods of pressing and sintering, it is very dimcult toobtain a uniform distribution of powder in thin layers, for examplelayers varying from a few thousandths of an inch up to 1 6 of an inchthick and to provide for even, uniform pressure. Furthermore, anymisalignment in a press or die results in a greater non-uniformity whenpressing thin sections than when pressing thick sections. Also there isa very high percentage of breakage in thin pressed sections when theyare transferred from the pressure die to the heat treating furnace. Thedifiiculties involved in the production of thin cemented carbidearticles are so great that heretofore such material has not beenavailable on a commercial scale.

It is one of the objects of the present invention to provide a simpleand relatively inexpensive process for the manufacture of thin metalplates or other articles from powdered material and particularly for themanufacture of thin cemented carbide articles such as strips and plates,as well as hollow cylindrical and cup-like articles having walls lessthan of an inch in thickness.

The novel features which are characteristic of my invention are setforth with particulity in the appended claims. My invention, itself,however, will best be understood from reference to the followingspecification when considered in connection with the accompanyingdrawing in which the single figure represents a cross section of a moldwhereby my invention may be carried into effect.

Referring more particularly to the drawing, I have indicated at l a moldwhich may be'made of graphite or other suitable refractory material suchas alumina and the like. The mold preferably has a fine grain and lowporosity and is provided with a channeled or cut away portion 2 thesurface 3 of which is very smooth and capable of takin a high polish.

In making thin walled articles I employ finely divided ingredients, forexample powdered material capable of passing through a standard 200 meshscreen and preferably through a standard 325 mesh screen. However, whenfor certain uses maximum physical properties are desired in the finishedproduct the fineness of the powdered inredients should not exceed 10microns in diameter and the greater number of grains preferably shouldbe less than 1 micron in diameter.

The powdered material is mixed with an oil to provide a paste having aputty like consistency. The oil employed should be volatile at atemperature substantially below the sintering temperature of theparticular cemented carbide being fabricated. Such volatilization alsoshould be accomplished without decomposition or appreciable action onthe powdered metal. In addition to being volatile with heat andnonacting with the powder, the oil should be capable of formin with thepowdered material an unctuous paste which may be spread uniformly in athin layer. An oil of this character is a commercially availablelubricating oil known as Carolina No. 675 which is completely volatileat about 400 F.

In producing a thin flat plate, a. layer 4 of the paste material isspread on the smooth polished surface 3 of the mold I. To provide for adefinite width and thickness of the plate and to permit easy spreadingof the paste to a uniform thickness the channeled part 2 of the mold hasside walls 5 the height of which is equal to the thickness of the pasteto be spread in the channeled portion of the mold. The paste may bespread with any suitable means, for example with a spatula or it may beextruded into the channeled portion 2 of the mold. The spreading actionmay be assisted by gently tapping or vibrating the mold l.

If desired, the paste material now may be presintered in the mold I-Under such circumstances there is somewhat of a tendency for thematerial being sintered to warp and break during the heat treatment.This difliculty may be the result of uneven removal of oil from theupper and lower layers of the paste. However, I have found that thetendency of the plate to break andwarp may be minimized and more uniformresults obtained if the upper surface of the paste 4 is covered with athin sheet 6 of paper or like material which in turn is covered by afiat graphite or other refractory plate 1. The paper may be pressed ontothe surface of the paste with a roller or other means. The plate 1 hassubstantially the same length and width as the mold l and'is applied tothe paper sheet as indicated on the accompanying drawing. This assemblyis then heat treated.

In the heat treating process the covered mold is placed in a furnaceprovided with a non-oxidizing atmosphere such as hydrogen and having alow temperature which is gradually raised to the desired temperature ofabout 1000 C. or if desired the loaded mold is passed slowly into theheated zone of the furnace. The charge in the mold is thus heated orpresintered at a temperature which will develop suflicient strength inthe charge to permit careful handling afte it has been cooled. Thetemperature to which the charge in the mold is heated at this point isbelow that at which a reaction will take place with the graphite sinceany such reaction will result in sticking, warping, carbon absorption,and other detrimental effects. During this heat treatment the oil withinthe charge is completely volatilized and the paper strip becomes charredand is removed easily.

After the plate 4 has been presintered the mold I or similar molds arecoated on the polished Surface 3 with a thin layer of a non-carbonaceousrefractory powder such as aluminum oxide and the presintered plateplaced on the oxide-coated surface and heated in a non-oxidizingatmosphere, for example hydrogen. The plate is gradually brought to atemperature sufiiciently high to provide maximum consolidation in thesintered plate. A composition consisting of about 91 tungsten carbideand 9% cobalt generally would be sintered at about 1400 to 1500 C. Thisfinal sintering temperature will vary somewhat depending upon the natureof the material being sintered but will be about equal to thetemperature employed with material which is pressed and sintered in thewell known manner. The final sintering is accompanied by a shrinkage inthe sintered material which should be uniform but which will vary withthe composition and physical condition of the starting metal powders.

Although I have illustrated my invention par ticularly in connectionwith cemented carbides, it is not limited to such material but may beapplied to the production of thin structures of any metal compositionwhich is customarily produced by sintering the powdered ingredientsthereof. Also, if desired, my process may be employed to advantage inthe manufacture of diamond impregnated cemented carbides since thediamonds may be positioned easily in the paste material. Also, ifdesired, the surface 2 of the mold may be provided with serrations toprovide the sintered plate during the presintering operation with a filelike surface.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is:

l. The method of making thin metal structureswhich comprises mixing themetal in finely divided condition with an oil to provide an unctuouspaste, said oil being volatile at a relatively low temperature withoutdecomposition and nonreactive with the powdered metal, spreading thepaste thus formed on a mold and in a layer of predetermined thickness,and thereafter sintering the powdered material in a non-oxidizingatmosphere.

2. The method of making thin cemented carbide structures which comprisesmixing the metal in finely divided condition with an oil to provide anunctuous paste, said oil being volatile at a relatively low temperaturewithout decomposition and non-reactive with the powdered metal,spreading the paste thus formed upon the polished surface of a mold, andthereafter heating the paste material at a temperature of about 1000 C.said mold having means for controlling the thickness of the pastematerial.

3. The method of making thin cemented car- .hde metal structures whichcomprises mixing the metal in finely divided condition with an oil toprovide an unctuous paste, said oil being volatile at a relatievly lowtemperature without decomposition and non-reactive with the powderedmetal, spreading the paste thus formed in a layer of predeterminedthickness on a mold, covering the surface of the paste material withpaper, placing a refractory plate on said paper and heating the assemblyto a temperature of about 1000 C.

4. The method of making thin cemented carbide structure which comprisesmixing the metal in finely divided condition with an oil to provide anunctuous paste, said oil bein volatile at a relatively low temperatureWithout decomposition and non-reactive with the powdered metal,spreading the paste thus formed in a layer of predetermined thickness ina cut-away portion of a graphite mold, said cut-away portion having apolished surface and heating the paste to a temperature of about 1000 C.to thereby volatilize the oil and pre-sinter the powdered metal.

5. The method of making a sintered cemented carbide metal plate whichcomprises placin a layer of metal paste consisting of a mixture ofpowdered metal and an oil volatile at a relatively low temperaturewithout decomposition and nonreactive with the powdered metal in achanneled portion of a graphite mold so that it fills but does notextend beyond said channeled portion, placing a thin sheet of paper onsaid paste, superimposing a flat graphite plate on said paper,

' heating the assembly in a reducing atmosphere at about 1000 C. andthereafter sintering the metal plate thus formed in a non-oxidizingatmosphere at a temperature of about 1400 to 1500 C.

6. The method of making a sintered cemented carbide metal plate whichcomprises placing in a channeled portion of a refractory mold a layer ofmetal paste consisting of a mixture of finely divided cemented carbideingredients and an oil which is volatile at about 400 F. withoutdecomposition and non-reactive with the cemented ingredients, said pastefilling but not extending beyond said channeled portion, superimposing arefractory plate on said mold so as to cover said paste without applyingpressure thereto. the surface of said plate opposite said paste beingflat and smooth, and thereafter heating the paste to volatilize the oiland sinter the powdered metal.

EDGAR W. ENGLE.

